As part of a study of the Vertebrata found in the Late Cretaceous (Early Maastrichtian to Middle Late Maastrichtian) Fox Hills Formation, 48 sites in western and central North Dakota were collected to interpret the chondrichthyan and osteichthyan paleofaunas. Based mostly on teeth, 19 shark species, 16 skate and ray species, and one ratfish species were recognized. Of those, three taxa are new, including Cretalamna feldmanni n. sp., “Myliobatis” foxhillsensis n. sp., and Dasyatis northdakotaensis n. sp. New chondrichthyan species occurrences for the Fox Hills Formation include: Squalus ballingsloevensis, Plicatoscyllium derameei, Cretorectolobus olsoni, Carcharias cf. C. tenuiplicatus, Cretalamna feldmanni n. sp., Paranomotodon toddi, Squalicorax pristodontus, Palaeogaleus navarroensis, Archaeotriakis rochelleae, Paraorthacodus andersoni, Synechodus turneri, Walteraja exigua, Dasyatis northdakotaensis n. sp., Rhombodus levis, “Myliobatis” foxhillsensis n. sp., and morphotypes of placoid scales and dermal denticles.
Twenty species of bony fishes were identified from teeth and other skeletal parts, two were vertebral morphospecies, two were based on scales, and four were recognized from otoliths. New osteichthyan occurrences in the Fox Hills Formation include: a lepisosteid, Melvius sp., Cyclurus fragosus, Protosphyraena sp., Belonostomus longirostris, Xiphactinus vetus, Paratarpon? sp., Pollerspoeckia siegsdorfensis, cf. Bathylagus sp., Enchodus cf. E. ferox, and “Apogonidarum” maastrichtiensis.
The Fox Hills Formation is Early Maastrichtian in Bowman County, southwestern North Dakota. The Bowman County sites yielded the oldest fossils of this study. Sites in the Fox Hills type area in north-central South Dakota and south-central North Dakota are Middle Late Maastrichtian based on the presence of Hoploscaphites nicolletii and Hoploscaphites nebrascensis Ammonite Zones and the Wodehousia spinata Pollen Zone. Age relationships of these fossil sites suggest temporal range extensions for several of the Fox Hills fish taxa.
Fox Hills fishes were derived from deep and shallow marine, brackish, and freshwater habitats. Five groupings were identified based on qualitative assessment of these habitat preferences. These groupings are: “offshore marine,” “nearshore marine,” “brackish water/estuarine–strong tidal influence,” “brackish water/estuarine–weak tidal influence,” and “riverine/lagoonal–strong freshwater influence.” Tooth morphology and comparison to modern analogs indicate presence of the following feeding types: omnivore, general invertebrate, molluscivore, pelagic piscivore, benthic piscivore, and scavenger. Species representing all feeding types occur in each of the five habitat groupings. Feeding competition was thus partitioned by habitat preference. When coupled with paleogeographic distribution information, the Fox Hills fish fauna indicates that some taxa represent a recurring assemblage of species that have a “large-river delta” habitat preference, as found today on major deltas of most continents.
Paleogeographic conditions in the Western Interior Seaway (WIS) were dominated by the physiographic conditions of the Hell Creek Delta and Dakota Isthmus complex, which is composed of lagoons, estuaries, and barrier island shorelines. The Fox Hills fish paleofauna includes taxa restricted to the WIS and those that also occurred in the Texas Gulf Coast, Mississippi Embayment, Atlantic Coastal Plain, Greenland, and Sweden. Pelagic, deep marine lamniform species were cosmopolitan and ranged to Europe
and North Africa. The Fox Hills fish fauna is most similar to the fish faunas of the Maastrichtian Kemp Formation, Texas, Severn Formation, Maryland, and Navesink and New Egypt formations, New Jersey.
The Fox Hills paleofauna documents fish extinction at the close of the Cretaceous. None of the 35 chondrichthyan species and none of the 20 osteichthyan species recovered from the Fox Hills Formation are found in the Paleocene worldwide. 58% of Fox Hills chondrichthyan and 77% of osteichthyan genera, and 20% of chondrichthyan and 33% of osteichthyan families, did
not survive after the Cretaceous. Support for this interpretation is provided by comparison of the Fox Hills paleofauna to the Paleocene Cannonball Formation paleofauna in North Dakota. None of the 13 Cannonball chondrichthyan species, nor any of
the four Cannonball osteichthyan species, occur in the Fox Hills Formation. Thirteen chondrichthyan genera (Squatina, Squalus, Ginglymostoma, Carcharias, Odontaspis, Cretalamna, Palaeogaleus, Galeorhinus, Paraothacodus, Synechodus, Myliobatis, Dasyatis, and Ischyodus) range across the K-Pg boundary.